Special Weather Discussion 2015-1-2

Severe Weather Discussion for April 8, 2015.
Issued: 04/06/2015.

Highlights:

– Active severe weather day is likely on Wednesday with the potential for significant impacts.

– Severe thunderstorms are possible across central Oklahoma on Tuesday, although the threat is highly conditional.

– Severe thunderstorms are likely Thursday across eastern Oklahoma, although the exact area depends on how fast this event evolves.

Discussion:

Forecast Changes – dropped central Oklahoma from Thursday. Little change to forecast reasoning other than adjusting to the speed offered by all models.

Tuesday – high surface based CAPE will be in-place Tuesday afternoon across a large part of central Oklahoma. Wind fields will be decent but not overly impressive. Additionally, models indicate only a small area where CIN may erode across western Oklahoma. This combined with neutral or even downward motion will keep chances limited tomorrow. Any storm(s) that do form will have the potential to produce hail to baseball size and winds to 70mph.

Wednesday – a much greater threat for severe weather is present on Wednesday. Impressive winds will translate eastward across the region overlaying a highly unstable airmass. Models point to dewpoints in the mid/upper 60s with deep layer moisture across the region characterized by 850mb dewpoints at least 12C. 18z NAM appears to shows a characteristic bias by sinking 850mb dewpoints on Wednesday. I have noted this problem for the past several years and it typically isn’t corrected until the night before. The other issue is that the 18z 4km NAM surges the dryline to near Oklahoma City by late Wednesday, placing storm initiation just west of HWY 81. I’m not sure if this surge is related to intense dryline mixing owing to limited 850mb moisture. Either way, the models are trending east with the dryline instead of a west, admittedly not expected. Storm motion looks to be rather quick at 35-40mph, so reaction time will be somewhat limited. A strong cap will keep storms down until mid-afternoon when height falls coupled with surface moisture convergence should breach said cap. I’m not convinced only a few storms will develop, I think more widespread development is possible. I also do not agree with the current SPC Day 3 and I’d think big changes are forthcoming on the upcoming Day 2. Either way, Wednesday continues to be a day to watch.

Thursday – all models shifted the dryline eastward early Thursday with the main impacts in far eastern Oklahoma. Clouds and rain from the night before could keep the threat on Thursday limited. For now, I’ll keep Thursday in the discussion.

Probabilities:

Risk area for any part of Oklahoma (valid: 04/07 Day 2, Day 3):

Slight: 100%, 100%
Enhanced: 100%, 75%
Moderate: 75%, 0%
High: 0%, N/A

Chase Status:

Level 3 – Wednesday appears to be the day.

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